Conventionally, a distributed control system (DCS) is established and an advanced automatic operation is implemented in a plant and a factory. In the distributed control system, field devices (for example, a measurement device and a manipulation device) and a control device controlling the field devices are connected to each other through a communication means. Although most of the field devices included in the distributed control system perform wire communication, a wireless field device which performs wireless communication in conformity with industrial wireless communication standards, such as ISA100.11a and WirelessHART (registered trademark), is realized in recent years.
An input/output unit, a wireless communicator, and a controller are disposed in a housing of the above-described wireless field device. The input/output unit generally performs measurement or operation of various state quantities (for example, pressure, temperature, and flow quantity). The wireless communicator performs wireless communication in conformity with the above-described industrial wireless communication standards. The controller totally controls operation of the wireless field device. It is not necessary to connect the wireless field device to a communication line or a communication bus like the conventional field device, and basically the wireless field device is independently installed in the plant. For this reason, a battery is often built in the wireless field device as a single power source.
The following patent document 1 discloses that a wireless module is attached to the conventional field device which does not have a wireless communicator, and the wireless module is capable of operating the conventional field device as a wireless field device. Specifically, the wireless module disclosed in the following patent document 1 includes an interface, a wireless communicator, and a power source. The interface is connected to the conventional field device. The wireless communicator performs wireless communication. The power source supplies electric power to the conventional field device through the interface. If a signal from the field device is input to the wireless module through the interface, the wireless module transmits the signal from the wireless communicator to a transmission destination (for example, a host controller). If the wireless communicator receives a signal of which destination is the field device, the wireless module outputs the received signal to the field device through the interface.